There is widespread interest in, and industrial need for, optical compositions and articles made therefrom having potential applications in the 1300 nm and 1550 nm telecommunications windows. Promising candidates for an efficient 1300 nm fiber optical amplifier material, for example, have included the rare earth ions Pr.sup.3+ and Dy.sup.3+ doped in fluoride, mixed halide, and sulfide glass hosts, while 1550 nm amplifier materials are suitably doped with Er.sup.3+. A recent publication by Borrelli et al., Transparent glass ceramics for 1300 nm amplifier applications, J. Appl. Phys. 78 (11), (Sept. 1995), reported an alternative host for the Pr.sup.3+ ion which combines some of the advantages of both fluoride and oxide glasses. The new material is described in the '505 patent and consists of an oxyfluoride glass which has been appropriately heat treated to form a transparent glass-ceramic. This glass-ceramic contained 5-40 volume % fluoride nanocrystals having diameters ranging between about 6-15 nm, embedded in a primarily oxide glass matrix. As described in detail in the '505 patent, optically active fluoride based glass-ceramic articles were produced from Yb-free compositions that included between about 50 to 900 ppmw Pr.sup.3+. The glass-ceramic was shown to perform as an active device in the 1300 nm spectral window over this dopant concentration range. For the Pr.sup.3+ doped glass-ceramics, fluorescence lifetimes greater than 120 microseconds were observed in the base glasses of the '505 patent at Pr.sup.3+ concentrations up to about 500 ppmw. Concentration quenching was observed at Pr.sup.3+ concentrations slightly above 500 ppmw, and fluorescence lifetimes were observed to decrease approximately linearly to about 70 microseconds at 900 ppmw. It was reported that a best case balance between fluorescence lifetime and concentration was achieved with Pr.sup.3+ in the range of about 200 to 550 ppmw; however, functional active devices were reported with Pr.sup.3+ concentrations in the range of about 50 to 650 ppmw. Since both longer fluorescence lifetimes and higher dopant concentrations are desirable for the production of active devices as described herein, the inventors recognized a need to improve upon the compositional ranges of the new glass-ceramic material described in the '505 patent, and to devise glass-ceramic compositions having similar advantages suitable for 1550 nm applications.
The radiative quantum efficiency is a key parameter in evaluating transparent glass-ceramics as a potential gain medium for fiber lasers and amplifiers. Quimby and Tick, in an article entitled Quantum efficiency of Pr.sup.3+ doped transparent glass-ceramics (to be published) report on the quantum efficiency of the 1300 nm emission in Pr.sup.3+ doped transparent glass-ceramics using a direct measurement technique based upon relative fluorescence measurements. Fluorescence was observed by exciting the Pr.sup.3+.sup.1 D.sub.2 level, peaking at around 1460 nm (the "A" transition), and the fluorescence from the .sup.1 G.sub.4 level, peaking at about 1300 nm (the "B" transition), when the .sup.1 D.sub.2 level was directly excited with 595 nm dye laser radiation. Following the analysis described by Quimby et al., Opt. Lett., 20, 2021 (1995), the quantum efficiency of the .sup.1 G.sub.4 1300 nm emission was determined by taking the ratio of the total B transition rate to the total A transition rate. The data in FIG. 1, to be described in more detail below, shows the measured B/A ratio for exemplary embodiments of the two base composition glass-ceramics of the '505 patent having Pr.sup.3+ concentrations ranging from about 25 ppmw to 1000 ppmw. As expected by the inventors, the B/A ratio increases with increasing concentration which they believe to be due to the effect of cross-relaxation resulting from increased Pr.sup.3+ ion clustering.
It is known that when a trivalent rare earth, e.g., Pr.sup.3+, is incorporated into these glass-ceramics, the rare earth is segregated into the second phase crystals which are formed during the ceraming process. These crystals have a cubic lattice structure and are thought to be comprised of mostly divalent cadmium- and lead-fluoride. The inventors believe that clustering arises from local strains that are established within the lattice because of the substitution of trivalent rare earth fluorides for the divalent fluorides. When direct substitution of a rare earth into the crystal lattice occurs, charge balance can be maintained by incorporating an interstitial fluorine into the crystal structure near the rare earth. In bulk crystals this is the source of the local strain, which is observed to decrease when these defects can cluster. The inventors believe that a similar mechanism occurs in the nanocrystals of the glass-ceramic. This, however, results in a decrease in the quantum efficiency at higher concentrations of Pr, observed by the authors to appear at concentrations of about 500 ppmw.
The inventors have therefore recognized a need for transparent rare earth doped glass and glass-ceramic compositions and articles made therefrom in which rare earth ion clustering and concentration quenching are reduced notwithstanding high rare earth dopant concentrations, which have a relatively high quantum efficiency, and a wider spectral gain band.